The present invention relates to a titanium film forming method and, more particularly, to a titanium film forming method of depositing titanium by chemical vapor deposition using a plasma, which can be applied to a semiconductor device manufacturing method including the step of forming a barrier metal.
With an increase in LSI integration degree, the diameter of a contact hole decreases, and the aspect ratio (depth/diameter) increases. Assume that interconnections are to be connected to the source and drain of a MOS transistor through contact holes having such a high aspect ratio. In this case, first, a silicide of a refractory metal such as titanium is formed at the interface doped with an impurity. Second, tungsten is used as a conductive material to be filled into the contact holes.
The first point will be described first. When, for example, aluminum is filled as a plug into a contact hole formed in an impurity region such as a source or drain in a silicon substrate, the aluminum is diffused in the silicon substrate. With an increase in LSI integration degree, impurity regions such as a source and drain are formed more shallowly. For this reason, an aluminum diffusion region reaches a portion deeper than an impurity diffusion region. If the plug material is diffused deeper than the impurity region in this manner, the function of the transistor is impaired.
If, for example, titanium silicide is formed on the bottom portion of the contact hole in advance, and the plug is formed on it, diffusion of the plug material into the silicon substrate can be prevented, thus solving the above problem.
The second point will be described next. A plug must be filled into a contact hole without producing any voids. It is not easy to fill a micropatterned contact hole having a high aspect ratio with a plug without producing any voids as described above. If, for example, aluminum is filled into the contact hole by sputtering, a void is produced in the central portion of the contact hole. For this reason, as is well known, tungsten is filled into the contact hole to form a plug by chemical vapor deposition (CVD). Since the CVD method exhibits excellent step coverage characteristics, the contact hole can be filled without any void. Tungsten is selected as a material which can be deposited by the CVD method and has a low resistance.
As described above, when contacts for connecting interconnections to the source and drain of a micropatterned MOS transistor, a refractory metal silicide such as titanium silicide is formed at the interface between each contact and the silicon substrate, and tungsten is formed on the silicide by the CVD method to fill each contact hole.
A conventional method of manufacturing a semiconductor device having contact holes formed in the above manner will be briefly described below.
The following is a case wherein contacts to be connected to a source and drain are formed.
First of all, as shown in FIG. 5A, silicon oxide is deposited on a silicon substrate 501, on which a MOS transistor is formed, so as to form an insulating interlayer 510. In this case, the MOS transistor is formed in a region partitioned by an element isolation oxide film 502 in the silicon substrate 501. This MOS transistor is made up of a gate electrode 504 formed through a gate insulating film 503 and a source and drain 505 formed by doping an impurity having a desired conductivity type into portions of the silicon substrate 501 which are located on the two sides of the gate electrode 504.
Subsequently, as shown in FIG. 5B, contact holes 511 are formed in the insulating interlayer 510 to expose the source and drain 505 formation regions.
As shown in FIG. 5C, a titanium film 506, which is a refractory metal film, is formed on the insulating interlayer 510 including the bottom portions and side surfaces of the contact holes 511 to have a thickness of about 10 nm. This titanium film 506 may be formed by depositing titanium by chemical vapor deposition using titanium tetrachloride, hydrogen, and argon as source gases. This deposition is performed while the silicon substrate 501 is heated to about 500.degree. C. With this process, the titanium film 506 and the silicon substrate 501 are made to react with each other to form a titanium silicide film 507 having a thickness of about 20 nm at the interface therebetween.
If tungsten is deposited on the titanium silicide film 507 to fill the contact holes 511, interconnections through the contact holes 511 can be formed. However, the deposition of tungsten reduces the titanium silicide film 507. A tungsten film is formed to fill the contact holes by CVD using WF.sub.6 as a source gas. In this formation of a tungsten film by CVD, since WF.sub.6 is used, the film formation atmosphere contains fluorine. Since this fluorine and titanium readily form a compound, and this titanium fluoride is a gas, the titanium content of this fluoride becomes lower than that of the titanium silicide film 507. That is, in forming a tungsten film by CVD, titanium silicide is etched. Since almost no titanium film 506 is left on the titanium silicide film 507, if a tungsten film is directly formed on the titanium silicide film 507 by CVD, the titanium silicide film 507 is reduced.
To prevent this, when titanium silicide is used as a barrier film, and a tungsten film is to be formed on the barrier film by CVD as described above, a titanium nitride film is formed to protect the titanium silicide.
First of all, to form this titanium nitride film on the titanium film 506, the titanium film 506 is exposed to ammonia to form a titanium nitride film 506a. This prevents a newly deposited titanium nitride film from peeling off. This is because a titanium nitride film formed on a titanium film tends to peel off.
With the above process, a new titanium nitride film 508 having a thickness of about 500 nm is formed on the titanium nitride film 506a having undergone transformation by nitriding, as shown in FIG. 5D. This film may be deposited by chemical vapor deposition using titanium tetrachloride, hydrogen, and argon as source gases.
A tungsten film is then formed on the resultant structure by CVD using WF.sub.6 as a source gas. Thereafter, the tungsten film and the titanium nitride films 506a and 508 on the insulating interlayer 510 are patterned to form interconnections 520 connected to the source and drain 505, as shown in FIG. 5E.
The process described above attaches importance to the thickness of the titanium film 506 which determines the thickness of the titanium silicide film 507 to a certain degree.
If the titanium film 506 is excessively thick, the titanium silicide film 507 also becomes thick. An increase in the thickness of the titanium silicide film 507 leads to an increase in consumption of silicon in the silicon substrate 501. If the titanium silicide film 507 is excessively thick, the bottom portion of the titanium silicide film 507 extends through the source and drain 505 formation region to come into contact with the silicon substrate 501. That is, if the titanium film 506 is excessively thick, the interconnections 520 cannot be properly connected to the source and drain 505.
In contrast to this, if the titanium film 506 is excessively thin, since the titanium silicide film 507 becomes thin, the resistance between the interconnections 520 and the source and drain 505 through the titanium nitride film 508 and the like increases.
The above titanium film is formed by plasma CVD using RF discharge based on titanium chloride gas as a source for the following reasons.
First of all, plasma CVD allows film formation at a temperature lower than that in thermal CVD, and also allows film formation at a proper deposition rate even by using a material that cannot react or react very slowly in a thermal process. According to plasma CVD, therefore, a thin titanium film can be formed while the formation of titanium oxide is suppressed. Titanium exhibits excellent corrosion resistance at temperatures around room temperature, but becomes very active and easily oxidizes at high temperatures. As compared with thermal CVD, plasma CVD exhibits excellent step coverage characteristics with respect to finer structures.
According to this CVD method using a plasma, film formation is performed in a reaction chamber forming a vacuum vessel in which a vacuum can be produced. More specifically, a substrate to be processed is placed in the reaction chamber, and the chamber is evacuated. Thereafter, titanium tetrachloride, hydrogen, and argon as source gases are fed into the reaction chamber, and argon plasma is generated by applying RF discharge into the reaction chamber while the substrate is heated. Upon generation of the plasma, the titanium tetrachloride is decomposed, and the resultant titanium is deposited on the substrate. In this case, the generated titanium is deposited on not only the substrate but also other portions in the reaction chamber.
Meanwhile, decomposed chlorine is generated to form a titanium film in the reaction chamber. If this generated chlorine is left in the reaction chamber, it reacts with titanium deposited in the reaction chamber to newly form titanium chloride. The presence of this generated titanium chloride causes excessive titanium chloride gas to be supplied to the substrate to be processed. In this state, the thickness of the titanium film 506 exceeds a desired value.
In a state wherein titanium is deposited in the reaction chamber, a titanium film is formed to have a thickness exceeding a desired thickness. If the settings are changed to decrease the thickness of this film, the formed films become excessively thin with an increase in the number of substrates to be processed. This is because, as the number of substrates on which thin titanium films are to be formed increases, the titanium deposited in the reaction chamber is consumed, the supply of excess titanium chloride gas decreases.
In this case, the resultant titanium film becomes thinner than a planned film. As a result, the obtained titanium film becomes excessively thin, and the titanium silicide layer becomes excessively thin.
As described above, when thin titanium films are to be formed by plasma CVD, the obtained thin titanium films vary in thickness depending on the number of substrates to be processed.
As described above, such variations in film thickness pose a serious problem when the thickness of each titanium film to be formed is to be accurately controlled.